Science | Science Experiment of the Week 13 | 331 - Bread Bubbles

Yeast, Sugar, Fungus, Carbon Dioxide, Enzymes, Starches - Look carefully at a slice of bread. What do you see? Lots of holes, right. How did all those holes get in the bread?

This Week's Experiment - #331 Bread Bubbles

These experiments are from Robert Krampf - The Happy Scientist

This week's experiment is about bread. If you have been on the list for long, you know that I like experiments with food, especially when it is tasty food. Lisa regularly bakes bread, and there are few smells that are better than the smell of freshly baked bread. To explore some of the science of this common food, you will need:

a slice of bread
a plastic sandwich bag
water
yeast
sugar

Look carefully at a slice of bread. What do you see? Lots of holes, right. Depending on the type of bread, the holes may be small or large, but it will have lots of open space. To get an idea of just how much empty space there is, squeeze the slice of bread into a ball. Get it as small as you can, which should be quite a bit smaller than it was when you started. How did all those holes get in the bread?

The answer is fungus. Fungus! Yes, fungus. When I was in school, fungus was grouped in the plant kingdom, since it was definitely not an animal. (That may give you an idea of how old I am.) Now the fungi have their own kingdom. It includes mold, mildew, mushrooms (yum!) and our focus this week, yeast. Yeast is made up of tiny, single celled fungi. These tiny organisms eat sugar. They also produce chemicals called enzymes which can break down starches into sugar. That lets them convert things like bread flour into sugar for food. As they consume the sugar, they give off carbon dioxide and alcohol. In bread making, the alcohol cooks away, although it does add some flavor to the bread. It is the carbon dioxide that we are mainly interested in this week.

To see the yeast in action, put about a teaspoon of yeast into a plastic sandwich bag. You want to be able to seal the bag airtight, so I used one of the zipper lock bags. You could also just tie a knot in the top of the bag to seal it. Before you seal it, you also want to add about three spoons of sugar and about half a cup of very warm water. You don't want the water to be hot, as that will kill the yeast. If you stick your finger into the water, it should feel warm but comfortable.

Once you have the water, yeast and sugar in the bag, squeeze out as much air as you can and then seal the bag. Place it in a warm spot and wait. After about 5 minutes, check on it. You should start to see tiny bubbles forming. After 15 minutes, you should see quite a few more bubbles. After an hour, the bag should be inflated quite a bit.

Now imagine that the yeast is inside some bread dough instead of the bag. As it changes the flour into sugar and consumes it, it gives off carbon dioxide and alcohol. In many bread recipes, you mix in the yeast and let the dough rise as the yeast produces gas bubbles. Then you "punch it down" which means that you squeeze the bubbles out and let it rise again. Now why would you do that! For the alcohol. Letting it rise twice gives it twice as much time to make alcohol, without getting too many gas bubbles. Your bread will not be alcoholic, but when the alcohol cooks away, it leaves behind flavors that add to the taste of the bread.

The kind of flour also makes a big difference for your bread. If dough is very stretchy and elastic, it traps the bubbles to give you a nice, light slice of bread. If the dough is less stretchy, then some of the gas escapes, and the bread will be denser and more solid, although it will still have lots of tiny holes. There are many kinds of flour, and each one makes bread with a different texture. I suggest that you compare several kinds, preferably with some butter and honey, or maybe toasted with some cheese, or maybe even some Vegemite, which is a tasty spread that is actually made from yeast. Not everyone likes the flavor, but I think it is quite good. I have even heard of a fellow that used a mixture of yeast and shoe polish every night before bed, so the next morning he could rise and shine. Sorry about that.....

Have a marvelous week.

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